In flat panel display devices such as digital cameras, mobile phones, personal digital assistants (PDAs), etc, in order to protect displays and enhance the appearance thereof, a thin plate-like cover glass is disposed on the front surface of the display so as to provide a broader region than an image display portion. Although the glass has a high theoretical strength, when scratched, its strength is largely lowered, and therefore, for the cover glass that is required to satisfy strength, a chemically strengthened glass having a compressive stress layer formed on the surface thereof through ion exchange or the like is used.
With the growing demand for weight reduction and thickness reduction in flat panel display devices, it is desired to thin cover glass itself. Accordingly, for satisfying the purpose, both the surfaces and the edge surfaces of cover glass are desired to have further strength.
For increasing the strength of the chemically strengthened glass, heretofore, a surface etching treatment to be conducted after chemical strengthening treatment is known (Patent Document 1).
Here, regarding the strength of glass, it is known that the strength of glass is lowered by the existence of hydrogen (water) in glass (Non-Patent Documents 1 and 2).